Tunnel or enameling pottery kiln



NOV 14, w M|LLAR TUNNEL OR ENAMELING POTTERY KILN Filed Nov. 15, 1929 2 Sheets-Sheet l I Invenz'or ag. cf i vflll ornjys Nov. 14, 1933. w. J. MILLAR 1,934,870

TUNNEL OR ENAMELING POTTERY KILN Fi l ed Nov. 15, 1929 2 Sheets-Sheet 2 2 7] [.lIlfllllllIl;

M E l J. W

Patented Nov. 14, 1933 TUNNEL OR ENAMELING POTTERY KILN William James Millar, London, England, assignor to Ajax Electric Company, Inc., Philadelphia, Pa., a corporation of Pennsylvania Application November 15, 1929, Serial No. 407,482, and in Great Britain November 21, 1928 1 1 Claims.

The invention consists in improvements, in tunnel kilns or furnaces heated by electric resistors inserted in the refractory walls.

These kilns may be of the single track type or there may be two tracks for trucks moving in opposite directions, so that heat is given up by the hot outgoing trucks to the cold incoming trucks.

Such kilns have considerable advantages over gas fired tunnel kilns in the cleaner atmosphere and the facility afforded for controlling the temperature at different parts of the tunnel.

For this purpose, according to the invention, the resistors are grouped and connected to the sources of supply so thatthe heat input can be adjusted to obtain the required variations of temperature both in the vertical and longitudinal direction of the tunnel.

Two independent main electric heating circuits are provided, one of which approximately suffices for the normal load and the other, of

smaller output, is automatically connected toa source of energy for the periods of time required to obtain predetermined temperatures at different parts of the tunnel.

The resistors are preferably wound in longitudinal grooves provided in the brickwork, the separate spirals being connected in parallel, so that adjacent turns are at the same voltage in order to reduce any danger of a short circuit etc.

The spirals arranged at different heights can be separately connected to the source of supply, so that the temperature gradient from the floor to the roof can be adjusted to suit the required conditions at any time, for example to suit the different temperatures necessary for firing articles decorated with colours requiring varying temperatures for firing.

The resistors may be connected to auto or other transformers by which the total input of heat canbe regulated according to the required working conditions.

Busbars may be provided in terminal boxes by which any resistor circuit can be connected to a direct current circuit or any phase of a polyphase supply circuit.

End chambers and transfer trucks are provided to enable the trucks to be inserted'in and removed from the kiln preferably through side doors provided in the chambers and closed during the heating process.-

The air pressure in the tunnel may be arranged to be slightly above atmospheric.

It is found in known electric pottery kilns, that water vapour or other gases force their way through the glaze on the ware and cause pin holes and other faults. These gaseous products having no free exit are condensed on colder parts of the ware and injuriously affect the glaze. The first drawback is prevented by correct distribution of the heat, so that the ware is not too rapidly heated.

To obviate the second drawback, in the kiln according to the invention the roof is raised where the temperature is' sufiiciently high to cause the evolution of the gases, and outlet fiues are provided for the escape of the gases. The. position of the inlet holes accordingly depends on the arrangement of the resistors. To assist the ventilation, inlet holes are provided on the 70 opposite side of the kiln at the level of the outgoing truck. A draught is'produced by these means which is directed upwardly and transversely and assists in the removal of the gases and also in the transference of heat from the hot to the'cold ware.-

The means for propelling the trucks preferably comprise a motor-operated variable speed gearing connected to areleasable clutch which slips at a given load, if, for example, a truck meets with an obstruction.

It is necessary that the operators at each end of the kilnshould work in synchronism, and for this'purpose signal lamps are provided which indicate the progress of the operations at the other end of the kiln.

In a two track tunnel kiln according to the invention resistors are preferably inserted in opposite walls of the middle portion of the tunnel, the resistors being relatively displaced on each side from the centre of the tunneltowards the inlet end of each track. A resistor set is also preferably arranged in the centre line of the tunnel at the middle of its length and brickwork partitions may be placed on this centre line at those parts where the resistors are provided only on one wall. Each resistor set may be separately controlled. a

In the accompanying drawings which represent a two-way kiln tunnel according to the invention- Fig. l is a sectional plan, and

Fig. 2 a section on the line 22 of Fig. 1, drawn to a larger scale.

Fig. 3 shows on a still larger scale the arrangement of the resistors in the central panels.

Fig. 4 shows the manner in which the brickwork of the roof is recessed.

Fig. 5 shows diagrammatically the electric circuits;

Fig. 6 shows diagrammatically the arrangement of the resistors in a vertical plane.

Figs. 7 and 8 show the position of the end chambers and propelling gear.

A are the outer walls of the tunnel consisting of building brick, B an inner wall of heat insulating material; C a heat insulating and heat resisting lining; D are partitions in the central line of the tunnel protecting the one side of the tunnel from the heat generated in the resistors placed on the opposite wall. The resistors E, E, E are wound in grooves c (Fig. 3) provided in the firebricks F. In the central panel the firebricks are provided with grooves on both sides and are supported by central vertical rods G of nickel-chrome or other heat resisting metal. H are outlet flues in the raised portion of the roof for the escape of any gases generated and I inlets provided with a damper i. The brickwork of the roof and side walls is preferably recessed at intervals as shown at J (Fig. 4) in order to prevent air draughts along the kiln when the end doors for the entrance and removal of the trucks are open.

Fig. 5 shows diagrammatically the circuit arrangements for heating the resistors. A threephase alternating supply is assumed in the exexample illustrated.

Auto-transformers K, K K are connected to the respective phases of the supply leads U and leads I, Z 1 connect tappings on the respective transformers over switches M, M M to the resistors E, E E

An auxiliary set of resistors N, N N is connected over switches O, 0 O to the leads 1, Z 1 and these resistors are automatically connected up by means of a thermostat P in the circuit of a battery Q and a relay q. If the temperature of the tunnel falls below a predetermined value, contacts p, p are closed, relay q is energized and closes a switch R which completes the circuit of a solenoid S by which the switches O, 0 O are closed and the resistors N, N, N connected to the supply source U.

Fig. 6 shows diagrammatically in sectional elevation how the main and auxiliary resistors E and N may be arranged on a side wall so as to obtain the required temperature gradient from the floor level upwards.

An excess number of resistors is provided, the kilowatts dissipated being a maximum in the lower ones, reducing to a minimum at the top. Those at the top occupy only a portion of the length occupied by those at the bottom, and when the kiln is developing its maximum kilowatt rating, a percentage of actual resistors will always be disconnected. In order to adjust the temperature gradient to meet varying requirements, both in the longitudinal and the vertical direction of the kiln, the most suitably proportioned and situated resistors are selected and connected to the auto-transformer. The tapping on the latter is selected to secure the k. w. load required.

Fig. 7 is an elevation and Fig. 8 a plan showing the doors provided for closing the side passages at each end of the kiln by which the trucks enter and leave the kiln and examples of gearing by which the trucks are propelled through the kiln.

The door W is balanced by a counterweight w.

The motor '1 is connected by gearing V to a pinion o engaging with a rack Y by which the tracks are propelled. Similar devices are provided at opposite ends in each tunnel of a double track kiln.

I claim:-

1. A truck tunnel kiln of the type in which the firing zone is heated by electric resistors, comprising side walls and groups of resistors arranged in said side walls at different vertical levels, supply lines and means for connecting, as required, certain of said groups of resistors to said supply lines, so that a required temperature gradient can s obtained both in the longitudinal and the vertical direction of the firing zone.

2. A truck tunnel kiln according to claim 1, in which the longitudinal dimensions of the resistors disposed at the lower levels are greater than those of the resistors at the upper levels, the applied voltage being correspondingly adjusted.

3. A truck tunnel kiln according to claim 1,

comprising two main independent resistor groups, one of which groups is connected to the supply lines in the normal use of the kiln, and the other of which groups is automatically connected to the supply lines as the temperature of the kiln falls below a predetermined point.

4. A truck tunnel kiln according to claim 1, comprising two main independent resistor groups, one of which groups is connected to the supply lines in the normal use of the kiln, a battery, a relay and contacts closed thermostatically in said battery circuit to energize said relay, and means by which said relay, when energized, connects the last mentioned groups of resistors to the supply lines.

5. A truck tunnel kiln according to claim 1, comprising a transformer connected to said supply lines, tappings on said transformers and switches for connecting said groups of resistors to said tappings.

6. A two-way truck tunnel kiln according to claim 1, in which resistors are provided in each of the side walls and are relatively displaced from the middle of the tunnel towards the inlet end of each track, and resistors are also placed in the center line of the tunnel at about the middie or the firing zone.

'7. A truck tunnel kiln according to claim 1, comprising a roof, a raised portion in said roof at the incoming truck side, air outlets in said raised portion, and air inlet dampers on the outgoing truck side, said air inlets and outlets being provided for the removal of any oily fumes gen erated.

3. A truck tunnel kiln according to claim 1 in which the heating eiTect of the upper resistors is less than that of the lower resistors and the upper resistors occupy a shorter kiln length than the lower resistors.

9. In a kiln, walls forming a heating passage, means for progressing charges through the passage, said means being adapted to position the charges at variant vertical levels within the passage, and a plurality of electric resistors at different vertical levels within the passage and dissipating heat at different rates, whereby a charge at one level will be subjected to a different temperature from a charge at another level.

10. In a kiln, walls forming a heating passage having considerable longitudinal extension, means for progressing charges through the passage, said means being adapted to position the charges at variant vertical levels within the passage and to maintain the level of a given charge constant during progression, and a plurality of resistors at different vertical levels within the tively variant vertical levels within the passage and to maintain a given object at the same level during progression, and a plurality of electric resistors at different vertical levels within the passage, dissipating heat at different rates which vary from a maximum at the bottom to a minimum at the top, whereby an object can be placed at a level selected in accordance with the desirable burning temperature of the colors involved, and, will be burned at the desirable temperature during its progression through the kiln.

WILLIAM JAMES MILLAR. 

